The organic EL display, composed of an array of organic electro-luminescence elements (organic EL elements), is one of the promising display devices from various advantages. Organic EL displays can be operated on reduced direct current voltages, and achieves high efficiency, namely, high brightness. Moreover, organic EL displays exhibit higher response speed than liquid crystal displays, and improved temperature characteristics at low temperatures. Additionally, organic EL displays achieve improved visibility through self light emitting, and therefore do not require backlights, differently from liquid crystal displays. The fact that no backlight is required preferably makes displays thinner.
Among passive and active matrix methods, which are known as driving methods of organic EL displays, the active matrix method is a promising driving method of organic EL displays. The active matrix method designates a method that involves providing TFTs (Thin Film Transistor) on display panels to activate organic EL elements. The active matrix method, which exhibits excellent light emission efficiency, is expected to achieve high image quality. Additionally, the active matrix method achieves extension of display lifetime through reduction in drive currents of organic EL elements,
Japanese Laid Open Patent Application (JP-A-Heisei, 11-282410) discloses a driving circuit of organic EL elements for an active matrix method. FIG. 1 is a circuit diagram of the disclosed driving circuit. The driving circuit is composed of n-channel transistors 131, 132, a capacitor element 133, and switches 134, 135. TFTs are used as the n-channel transistors 131, 132. The n-channel transistors 131, and 132 form a current mirror. This current mirror is supplied with a signal current corresponding to the brightness of an organic EL element 103 from a signal line 104. The current mirror generates a drive current for driving the organic EL element 103 in response to the supplied signal current.
One problem of this driving circuit is that the generated drive current may not correspond to the signal current due to the non-uniformity of the properties of the n-channel transistors 131, and 132. TFTs, which are used as the n-channel transistors 131, 132, tend to exhibit increased non-uniformity, differently from transistors integrated within a silicon single crystal; TFTs suffer from increased variance in carrier mobilities, film thicknesses of gate dielectrics, and threshold voltages. Even for a pair of adjacent TFTs, the threshold voltages thereof may be different by about several 10 mV. The variation in the properties of the n-channel transistors 131, and 132 may cause non-uniformity in brightnesses of a plurality of organic EL elements 103, that is, a plurality of pixels, for the same signal current to be supplied thereto. This undesirably deteriorates the image quality of the display panel.
Another problem of the driving circuit is that the drive current supplied to the organic EL element 103 may be nonlinear with respect to the signal current, because of a parasitic capacitance 136 of the signal line 104. A part of the signal current is used to charge the parasitic capacitance 136 of the signal line 104, and not supplied to the current mirror. The part of the signal current used to charge the parasitic capacitance 136 does not contribute to the driving current. For a minute signal current, in particular, the part exhausted to charge the parasitic capacitance 136 cannot be ignored. As a result, as shown in FIG. 2, the drive current Iout exhibits nonlinearity with respect to a signal current Iin. The nonlinearity of the drive current Iout prevents a low grayscale level from being accurately represented, and degrades the image quality of the display panel.
The influence of the nonlinearity of the drive current Iout is important to the active matrix method in which the signal current is relatively small. In the active matrix method, the drive current supplied to the organic EL elements ranges between 1/50 and 1/100 of the drive current of the passive matrix method; the minimum drive current ranges between several nA and ten-odd nA. The small drive current may cause a trouble that the charging of the parasitic capacitance 136 is not completed in one frame, which disables the pixels to represent the low brightness.
Japanese Laid Open Patent Application (JP-A-Heisei, 5-35202) discloses a drive circuit adapted to the active matrix method, in order to suppress the influence caused by the variation in properties of elements. Nevertheless, the drive circuit is adapted to drive a voltage-driven liquid crystal; the drive circuit does not solve the above-mentioned problems.